1. Technical Field
The present disclosure relates to a light-condensing unit, a solid-state image sensor, and an image capture device including the solid-state image sensor.
2. Description of the Related Art
A solid-state image sensor is made by forming a huge number of photodetectors in a silicon (Si) substrate. The solid-state image sensor generates an electrical signal representing the quantity of light that has been incident on each of those photodetectors by photoelectric conversion. The photodetectors are typically photodiodes formed in the silicon substrate. The light that has been incident on each photodetector is absorbed into silicon to generate electron-hole pairs.
FIG. 30 is a graph showing how the ratio of the light that has been incident perpendicularly onto the surface of silicon and then absorbed into a region to a depth of 4 μm under the surface of silicon (i.e., the absorbed fraction) changes with the wavelength of the incident light. In this graph, the abscissa represents the wavelength of the incident light and the ordinate represents the absorbed fraction. As can be seen from FIG. 30, light with the color blue wavelength (of 400 to 500 nm) is absorbed almost 100% into that region to the depth of 4 μm under the surface. However, the longer the wavelength of the incident light, the lower the absorbed fraction. And only 10 to 20% of near-infrared light (with a wavelength of 800 to 900 nm) is absorbed. That is to say, an infrared ray (which will be referred to herein as “infrared light”) is hardly absorbed into the surface region of silicon but is transmitted to reach a region deep under the surface.
In this description, the visible light wavelength range is defined to be from 400 nm to 700 nm, and the infrared light wavelength range is defined to be from 700 nm to 2.5 μm. Also, the infrared light will be sometimes abbreviated herein as “IR light”.
A conventional solid-state image sensor that uses such a property is disclosed in Japanese Laid-Open Patent Publication No. 2009-272620. The solid-state image sensor disclosed in Japanese Laid-Open Patent Publication No. 2009-272620 includes a lot of photodetectors (visible light detectors) which are arranged two-dimensionally on an image capturing plane and infrared light detectors which are arranged deeper than the visible light detectors with respect to the image capturing plane. These photodetectors are implemented as respectively independent photodiodes and generate electrical signals representing the quantities of light absorbed by photoelectric conversion.
The infrared light that has been incident on the image capturing plane of this solid-state image sensor is not absorbed into the visible light detectors but reaches the infrared light detectors which are located deeper than the visible light detectors. Meanwhile, most of the visible light is absorbed into the visible light detectors and does not reach the infrared light detectors. With such a solid-state image sensor, the photosensitive area of the infrared light detectors can be set to be sufficiently broad without being constrained by the photosensitive area of the visible light detectors.